Method and apparatus for fastening an auxiliary joining element and work piece

ABSTRACT

An auxiliary joining element is joint to a sheet metal work-piece with a foot deformed to interlock the auxiliary joining element to the sheet metal work-piece. The foot is depressed into the sheet metal work-piece, which is retained over a die having a recess defined by a wall. The wall is interrupted by die parts that are movable radially inwardly of the wall. The foot deforms the sheet metal work-piece into the recess and simultaneously forces the die parts radially inwardly of the wall. Spaced undercuts are formed by the die parts in the foot and the deformed sheet metal work-piece for retaining the auxiliary joining element to the sheet metal work-piece.

The invention relates to a method of fastening an auxiliary joiningelement to a sheet-metal-like work-piece in which a foot of theauxiliary joining element is pressed from one side into the work-pieceand deforms this in pot-like manner. Furthermore, the invention relatesto an apparatus for the fastening of an auxiliary joining element to asheet-metal-like work-piece having a die which has a recess and having aholder for the auxiliary joining element which is arranged in alignmentwith the recess and can be moved in a pressing direction relative to thedie. Finally the invention relates to a work-piece having an auxiliaryjoining element in which the work-piece has an outwardly directedprojection into which a foot of the auxiliary joining element projects.

The term “sheet-metal-like work-piece” in connection with the presentinvention not only means a piece of sheet metal but rather work-pieceswhich are formed, at least in the region of the auxiliary joiningelement in plate-like manner with a relatively small wall thickness,with the material of the work-piece being capable of being deformed toan adequate degree. In addition to pieces of sheet-metal plastic panelsalso fall under the term of a sheet-metal-like work-piece.

In some cases it is necessary to connect an auxiliary joining elementwith the work-piece in order, with the aid of the auxiliary joiningelement, to be able to attach third elements to the work-piece.Auxiliary joining elements can for example be threaded bolts which havean external thread onto which a nut having a thread can be screwed.Auxiliary joining elements can also have an internal thread into which abolt can be screwed. This listing is however not exclusive. Auxiliaryjoining elements are required in large numbers in motor vehicles anddomestic appliances for the attachment and holding of trim panels andconducting lines. Grooving or roughness on their surface is frequentlysufficient for the attachment of other parts.

Such auxiliary joining element having the advantage that they can beconnected to the work-piece without the supply of heat being necessaryfor this purpose, as is the case with welding or solding for example.Other aids, such as adhesives are not necessary. Accordingly theconnection of the auxiliary joining element to the work-piece by formingis always of advantage when different materials are used for thework-piece and the auxiliary joining element which cannot otherwise beconnected without further ado.

In the simplest case the auxiliary joining element is pressed into thework-piece and deforms the latter in such a way that it has, at the sideopposite to the auxiliary joining element, a pot-like or beaker-likeoutwardly directed projection. This auxiliary joining element is thenclampingly held in the work-piece. A connection of this kind has indeedin most cases adequate shear strength. The resistance against headpull-out and the security against rotation are however restricted.

DE 30 03 908 A1 shows a stud having piercing and riveting behaviour.This stud produces an opening by piercing on being inserted. The slugwhich results is retained in a recess at the underside of the stud. Theperipheral wall of this recess is bent radially outwardly towards theend of the piercing process and then engages around a likewise bent-overedge of the work-piece which has arisen during the piercing process.

DE 22 44 945 A1 shows a method for the mechanical connection ofsleeve-like parts with plate-like parts in which the plate-like partmust be pierced prior to the introduction of the sleeve-like part. Thesleeve-like part has in this respect various “feet” which have to bebent outwardly after insertion.

DE 196 47 831 A1 shows a method for the attachment of a functionalelement, for example a stud, to a sheet-metal work-piece in which thefoot of the stud has projections and recesses through which undercuts ofthe work-piece form with the foot of the stud during the insertion.

The invention is based on the object of securing an auxiliary joiningelement which can be loaded in several directions to a work-piece in asimple manner.

This object is satisfied in a method of the initially named kind in thatthe foot forms an undercut with the work-piece and the undercut isrestricted to predetermined peripheral regions.

With this manner of proceeding one not only presses the auxiliaryjoining element into the work-piece, with the work-piece being deformed.The auxiliary joining element is also deformed in this way at its foot.The foot is deformed at least partially radially outwardly in relationto the peripheral direction and thereby forms an undercut to thework-piece, which is likewise deformed radially outwardly in theseregions. “Radial” here relates to the main axis of the auxiliary joiningelement, for example its thread axis. With this design one achieves onthe one hand an improved strength against head pull-out. The auxiliaryjoining element is secured more strongly against being pulled-out of thework-piece. As a result of the interrupted undercut in the peripheraldirection care it is also ensured that the auxiliary joining element issecured in the work-piece against rotation. Thus, the screwing on ofnuts or the screwing of bolts into the auxiliary joining element is suchscrew connections can be pulled tight with a relatively high torque.Finally this design has the advantage that one obtains a connectionbetween the auxiliary joining element and the work-piece in which theauxiliary joining elements and the work-piece contact one another with arelatively high pressure, in particular of the region of the undercuts.This pressure remains even after completion of the connection. This isin particular favourable when an electrical current is to be transferredvia the auxiliary joining element to the work-piece, for example, whenthe auxiliary joining element is used as a ground connection bolt in thesheet metal of a vehicle body.

Preferably one allows material to flow from regions without undercutinto regions with undercut. For the manufacture of the undercut regionsthere is now more material available. One can, in other words, nowconcentrate the material which is normally available at the entireperiphery of the pot-like outwardly deformed projection into a fewundercut regions. Thus it is possible, with the same quantity ofmaterial, to allow the undercut to project further or deeperperpendicular to the pressing direction. One has found that the strengthof the connection is dependent to a stronger degree on the depth of theundercuts than it is on the length in the peripheral direction. Thus, ifone restricts the undercuts to regions in the peripheral direction theseregions can be designed with greater overlap in the undercut region andthe connection as a whole is then stronger and indeed both with respectto the head pull-out strength and also with respect to the securityagainst rotation.

One preferably produces wall sections which extend parallel to thepressing direction at the outer side of the work-piece which liesopposite to the auxiliary joining element. This design have severaladvantages. On the one hand the demolding, i.e. the extraction of thework-piece provided with the auxiliary joining element out of thecorresponding apparatus, for example out of a die, is relatively simple.In the regions where the outer side extends parallel to the pressingdirection one then no longer has to perform any deformation work inorder to remove the work-piece. It is only necessary to overcome theadhesive friction forces. On the other hand, one can, particularly withat least approximately vertical peripheral walls, ensure that ideal flowpaths into the undercut regions are present for the two materials of thework-piece and the auxiliary joining part.

One advantageously produces a closing force on at least one tool partwhen pressing via the work-piece and an opening force on the tool partwhich is arranged in the region of an undercut on extracting the unitformed by the work-piece and the auxiliary joining element. The methodis thus quasi self-controlling. In the region of the recess there islocated a tool part which is so formed that an undercut arises when thematerial of the work-piece is pressed in there. Since this tool part isheld by the work-piece in its closed position the tool part cannot open.The situation is however different when the work-piece is extracted fromthe tool. In this case the pressure on the tool part drops away. Thiscan then open and releases the work-piece. For the opening a very smallforce is required. During the open process no reverse deformation canaccordingly be brought about.

Preferably three or more undercut peripheral regions are produced. Inthis way a connection can be achieved which is supported on all sidesperpendicular to the tensile force. The more undercut regions that arepresent the better is the security against rotation.

One preferably applies the pressure to the auxiliary joining element atan auxiliary shoulder. This is in particular of advantage when theauxiliary joining element is provided with a thread. The auxiliaryshoulder is then so positioned that the thread is not compressed, ordeformed in another manner, when applying the pressure.

The auxiliary shoulder is preferably arranged adjacent to thework-piece. Thus only a very short length of the auxiliary joiningelement is available in which the auxiliary joining element can bedeformed. In the remaining length a region can then be provided whichreceives the thread, which in this embodiment can, intentionally, nolonger be deformed.

In a preferred embodiment provision is made that at least one furthersheet-metal-like work-piece is arranged between the work-piece and theauxiliary joining element and is likewise of a pot-like shape withundercuts restricted in the peripheral direction. One can simultaneouslyuse the auxiliary joining element in order to produce a through clinchedconnection. The auxiliary joining element then forms a “lost plunger”i.e. additionally secures the through clinched connection againstrelease. For the further work-piece the same applies with respect to thenature of the sheet-metal part as for the above-named first work-piece.It need not, however, be of the same material. For example, one can inthis manner connect a piece of sheet-metal with a plastic part and theauxiliary joining element. The connection of the two sheet-metal-likecomponents additionally has an extraordinarily high shear force and headpull-out strength. Moreover, the connection of the two components issealed because no openings or cut joints are produced.

The object is satisfied by an apparatus of the initially named kind andin that the peripheral wall of the recess has wall sections which arearranged on levers, with the levers being movable by pressure in thepressing direction to a working position and being fixable there andforming undercut regions and being movable by a movement of the unitcomprising work-piece and auxiliary joining element opposite to thepressing direction into a release position in which the undercut regionsare fully released.

With a joining apparatus of this kind one first of all obtains arelatively simple layout of the die. Through the use of levers orfingers which can be brought into their working position and held thereby the pressing process itself one saves auxiliary aids such as springsor other pre-stressing means which are required in order to place thedie in the closed state, which one requires in order to be able to startthe deformation at all. The levers move into their working position atthe instant where the tool is pressed onto the die and is subjected topressure via the auxiliary joining element; they are thus moved radiallyinwardly, i.e. pivoted and then make available undercut regions. Theycannot move out of this working position and indeed also not under thepressure of the inflowing material because they are held in the workingposition by the work-piece itself. The undercuts formed by the leversnow make a space available into which the material of the work-piece andof the foot of the auxiliary joining element can flow. In thisconnection one can assume that not only the material of the work-pieceflows into the undercut region, but rather also the material of the footof the auxiliary joining element, so that the auxiliary joining elementforms an undercut with the work-piece in the sense of a form-fittedhooked engagement. With an undercut of this kind, which can also berecognized at the die side, the extraction of the work-piece from thedie would normally signify a certain problem. In accordance with theinvention this problem does not however arise because on lifting thework-piece the corresponding lever is moved outwardly, i.e. pivoted, sothat it can enter into the release position where it fully releases thework-piece. In this connection the lever only has to overcome smallspring forces, so that the extraction of the work-piece can take placewith relatively little effort. The fact that, on removing the work-piecefrom the die, the levers do not stretch the underside of the work-pieceunder pressure, so that corresponding tracks can be largely avoided,comes as a further advantage. This not only protects the work-piece butrather also the corresponding contact surfaces on the levers.

The levers preferably have a substantially planar top side which in theworking position stand perpendicular to the pressing direction and liesin the same plane as the top side of the die. Thus the pressing forceacts in such a way that the levers are only loaded in the closingdirection. The levers do not have to bear any lateral forces. Becausethe work-piece, so to say, sees an oppositely continuous and planarsurface, if one ignores the recess, then no markings arise in thesurface of the work-piece outside of the connection zone. Pressure peakson the levers can be avoided. The loading takes places relativelyuniformly in the working position so that the levers are protected andaccordingly have a relatively long working life.

Each lever is preferably formed as a cranked lever. The pressing forcewhich is used for the movement of the levers into the working positionand for the holding of the levers can then act on a larger area. Thelever transmission ratios are more favourable here so that one can alsobear the required forces with a relatively weakly dimensioned lever.

In a preferred manner the cranked lever has a short arm on which thewall section is arranged and a long arm at which a pivot axis or a pivotregion is located. The lever is thus formed in the manner of anupside-down L, can however also have further projections or recesses.The wall section which forms a part of the side-wall of the recess ofthe die and thus the undercut regions is located at the end face of theshort limb. The forces which act here are passed on by a relatively longlever arm to the pivot axis or to the pivot region. The term “pivotregion” gives expression to the fact that the pivot axis can move alongthe lever. If one now allows the closing forces to act via a similarlylong lever arm, i.e. on the outer side of the short limb of the “L” thena good force equilibrium occurs with a relatively small degree of costand complexity.

In an alternative embodiment the lever has an outer side which includesan acute angle with the pressing direction and is guided on acorrespondingly inclined counter-surface which extends outwardlyopposite to the pressing direction. This counter-surface is arranged inthe housing in which the die is arranged. If now the pressing force actsvia the work-piece onto the lever then the lever is displaced on theinclined surface in the pressing direction and correspondingly movesradially inwardly so that it forms an undercut region. The lever can notdeviate upwardly or radially outwardly even when the material ofwork-piece is forced radially outwardly because it is held firm radiallyfrom the outside by the counter-surface and a movement upwardly isprevented by the contacting work-piece. Depending on how much thecounter-surface and the outer side of the lever arm are inclined, veryhigh contact pressure forces can be achieved here so thatcorrespondingly high forces can also be applied for the forming of thework-piece.

At least three levers are preferably arranged distributed in theperipheral direction of the recess. With a uniform arrangement of thiskind one can expect a connection of the auxiliary joining element withthe work-piece which can be uniformly loaded in all directions.

Stationary wall sections which extend substantially parallel to thepressing direction are preferably provided between the movable wallsections. In this way only individual sections result along the wall ofthe recess of the die in which an undercut is present. In the remainingwall sections a cylindrical form of the work-piece results at the sideopposite to the auxiliary joining element. This results in a securityagainst rotation with a relatively high strength. The demolding, i.e.the removal of the work-piece from the die is simplified. In the regionswhere the wall sections extend parallel to the pressing direction onecan simply pull the work-piece opposite to the pressing direction out ofthe die. Only in the remaining regions it is necessary for the levers topivot outwardly. A further advantage lies in the fact that, as explainedin connection with the method, more material is available for theformation of the undercut. In this way it is possible to allow theundercut overlap to become larger outwardly, i.e. perpendicular to thepressing direction. This possibility results from the fact that materialcan be displaced from the regions with stationary wall sections into theundercut regions.

The die preferably has a security against drop-out for each lever. Thesecurity against drop-out has two advantages. On the one hand, on themoving a work-piece from the die one no longer needs to pay attention tothe fact that the levers remain in the die. These are rather held by thesecurity against drop-out. Oh the other hand one can now use the die inan “overhead” position, i.e. the auxiliary joining element can bebrought into engagement with the work-piece from below as seen in thedirection of gravity. In this way one obtains in a higher degree offlexibility with respect to the installation position in the operationof the apparatus.

The plunger preferably has a recess into which a shaft of the auxiliaryjoining part projects and which is surrounded by a pressure surface,with the auxiliary joining element having an auxiliary shoulder whichlies on the pressing surface. In this way one can achieve a situation inwhich the auxiliary joining element is admissibly guided by the plunger.The loading can however be restricted to regions of the auxiliaryjoining element which lie outside of the guide and thus of the plunger.An impermissible deformation of the auxiliary joining element is avoidedin this way, particularly when using threads on the auxiliary joiningelement.

The object is satisfied by a work-piece with an auxiliary joiningelement in that the foot forms an undercut with the work-piece and theundercut is restricted in the peripheral direction to predeterminedperipheral regions. In this way one can achieve a situation in which arelatively high security against rotation is achieved. In addition thedepth of the undercut, i.e. the depth of the form-locked hookedarrangement, can be made relatively large. The material required forthis can originate from the regions in which no undercut is present. Theflow characteristics, and also the combination of the auxiliary joiningelement and the work-piece can be optimized through the forming at theactive surfaces of the levers forming the undercut. The size and thelocation of the form-fitted hooked connection can be optimized anddefined by the choice of the predetermined peripheral regions and thedepth of the undercut.

The invention will be described in more detail in the following withreference to preferred embodiments in conjunction with the drawing inwhich are shown:

FIG. 1 a schematic view of an apparatus for the connection of anauxiliary joining element to a sheet-metal-like work-piece,

FIG. 2 a plan view of a die of the apparatus of FIG. 1,

FIG. 3 an auxiliary joining element,

FIG. 4 another embodiment of an auxiliary joining element,

FIG. 5 a work-piece with auxiliary joining elements secured therein,

FIG. 6 a schematic representation of a connection of twosheet-metal-like components with the aid of auxiliary joining elementsand

FIG. 7 a representation corresponding to FIG. 1 with another auxiliaryjoining element,

FIG. 8 an apparatus which has been modified relative to FIG. 1,

FIG. 9 a view corresponding to FIG. 2, and

FIGS. 10 a–10 d various stages while operating with the apparatus ofFIG. 8

FIG. 1 shows an apparatus for the connection of an auxiliary joiningelement 2 to a sheet-metal-like work-piece 3.

The sheet-metal-like work-piece 3 can also be formed from sheet-metal.It can however also be a work-piece of plate-like form, at leastsection-wise of a plastic material which can be deformed similarly tometal sheets.

Examples for auxiliary joining elements are shown in FIGS. 3 and 4 withthe auxiliary joining element of FIG. 3 corresponding to that of FIG. 1.

The auxiliary joining element 3 which is shown in FIG. 3 comprises ashaft 4 with an external thread 5 and thus forms a threaded bolt. Theshaft 4 is connected to a foot 6 which has a somewhat larger diameterthan the shaft 4. The difference in diameter forms an auxiliary shoulder7. The foot 6 has a substantial cylindrical outer periphery. At its baseside it has a recess 8 which facilitates a deformation, as will be laterexplained in conjunction with the method of the proceeding.

FIG. 4 shows an alternative embodiment of an auxiliary joining element2′ in which the same parts are provided with the same referencenumerals. The shaft 4′ has in this case an inner thread 5′ so that theauxiliary joining element 2′ forms a nut element into which a screw canbe inserted in order to connect the screw to the work-piece 3.

Instead of the two illustrated auxiliary joining elements 2, 2′ otherauxiliary joining elements can naturally also be used so long as thesehave a fastening profile, for example one or more grooves and/or projectout of the work-piece 3 in such a way that they still make a fasteningsurface available.

The apparatus 1 with which the auxiliary joining element 2 is connectedto the work-piece 3 has a plunger 9 which contains a bore 10 into whichthe auxiliary joining element 2 can be inserted. In this arrangement thediameter of the bore 10 is matched to the outer diameter of the shaft 4so that the auxiliary joining element 2 is received with littleclearance in the plunger 9. A pressing surface 11 is arranged around thebore 10 with which the plunger 9 presses onto the auxiliary shoulder 7of the auxiliary joining element 2. The pressing surface 11 is arrangedin a recess 12 which is surrounded by a peripheral boundary 13. Theplunger 9 is movable in the direction of a double arrow 14. Theorientation of FIG. 1 will be used for the subsequent explanation, i.e.the plunger 9 is movable upwardly and downwardly. The pressing directionis directed downwardly.

The apparatus 1 furthermore has a die 15 which is arranged in a dieholder 16. The work-piece 3 can be pressed into the die 15 with the aidof hold-down members 17 which stand under the action of springs 18.

The die 15 has a recess 19 the basic shape of which is cylindrical. Therecess 19 is thus surrounded by wall sections 20 which extend parallelto the pressing direction 14 and have a form of a cylindrical jacketsurface. At the base of the recess 19 a step-like projection 21 isprovided which, as will be explained further below, facilitates theflowing of the material during the connection of the auxiliary joiningelement 2 and the work-piece 3.

The cylindrical outer wall 20 is interrupted by levers 22. One suchlever is shown in its working position in FIG. 1. In this position itsupper side 33 lies flush with the upper side of the die 15.

The lever 22 has the shape of an inverted L with a short limb 23 and along limb 24. In the working position it lies with both limbs in contactwith the die 15, i.e. it is completely supported.

The short limb 23 forms with its end face 25 a part of the peripheralwall of the recess 19. In the working position this end face 25 isinclined, i.e. opens downwardly somewhat so that the material of thework-piece 3 can flow into the free space 26 which is formed by theinclined end face 25 of the lever 22.

In the peripheral direction four levers 22 are provided which interruptthe cylindrical outer wall 20 of the recess 19. Accordingly, on pressingthe auxiliary joining element 2 into the work-piece 3 a total of fourundercut regions are produced.

As can be seen from FIG. 1 the lever 22 is movably mounted in the die15. It has at its “rear side”, i.e. the side which is remote from therecess 19, a shoulder 26, so that it can be lifted in the die 15 untilthe shoulder 27 comes into contact with the projection 28 of the die.The projection 28 thus forms, together with the shoulder 27, a securityagainst drop-out. The projection 28 also serves as a turning or tiltingpoint for the lever 22 with the lever 22 being able to slide outwardlyalong the projection 28 during tilting.

On lifting of the lever 22 the lever 22 can swing outwardly, i.e. theend face 25 can move upwardly and simultaneously outwardly so that theshort limb 23 of the lever 22 fully frees the recess 19, so that thework-piece 3 can b removed from the die 15.

For the attachment of the auxiliary joining element 2 in the work-piece3 the auxiliary joining element 2 is inserted into the recess 10 of theplunger 9. The work-piece 3 is held tight on the die 15 with the aid ofthe hold-down members 17. This starting position is shown in the righthand half of FIG. 1.

The plunger 9 is then lowered onto the die 15. The plunger 9 presses theauxiliary joining element downwardly at its auxiliary shoulder 7. Inthis way the work-piece 3 is deformed. In the sections of the recess 19which are restricted by the cylinder wall 20 a beaker-like or pot-likeoutwardly directed projection is produced. As a result of the inclinedend face 25 of the lever 22 a part of the material however also flowsinto the free space 26 and thus forms undercuts 29, as can be seen inFIG. 5.

With this deformation process the foot 6 with the auxiliary joiningelement 2 is also deformed. In the region of the undercuts 29 the foot 6forms undercuts 30 with the work-piece 3. This deformation is assistedby the projection 21, which presses the material of the work-piece 3into the recess of the foot 6 of the auxiliary joining element 2 andensures, on continuation of the connection process, that the material ofthe foot 6 forms the undercuts 30.

Since the plunger 9 only acts on the auxiliary shoulder 7 of theauxiliary joining element 2 the thread 5 is not deformed. The pressingmovement ceases when the peripherally extending wall 13 of the plunger 9comes into contact with the work-piece 3.

Since the undercuts 29, 30 are not continuous in the peripheraldirection, but rather interrupted, it is possible to allow material toflow from the cylindrical sections into the undercuts 29, 30, so thathere the depth of undercut can be increased.

It can be seen from FIG. 5 that the auxiliary joining element 2′ formedas a nut (FIG. 4) can be inserted into the work-piece 3 in a similarmanner to the auxiliary joining element 2 formed as a bolt. In bothcases the corresponding undercuts 30, 30′ at the foot 6, 6′ resultswhich cooperate with corresponding undercuts 29, 29′ at the work-piece3. Thus not only does a high head pull-out strength and shear strengthresult in the connection of the auxiliary joining element 2, 2′ in thework-piece 3, but rather a relatively high resistance against rotationresults from the fact that the undercuts are restricted in theperipheral direction.

The illustrated auxiliary joining elements 2, 2′ can also be formedwithout a thread. With a bolt grooves or other measures by which thesurface can be made with higher grip can also be sufficient if required.With auxiliary joining elements of this kind it is sufficient in manycases to set a clamping element in place in order to secure a part to befastened at the auxiliary joining element 2. A similar situation appliesfor the auxiliary joining elements 2′ which are shown at the right inFIGS. 4 and 5 in which an internal thread 5′ is not absolutelyessential.

In the right hand half of FIG. 5 it can be seen that the connection ofthe auxiliary joining element 2′ formed as a nut forms an additionaladvantage. A sealed connection namely results between the nut and thework-piece 3, so that no additional measures are necessary for a seal.

FIG. 6 shows a connection of the auxiliary joining elements 2, 2′corresponding to the illustrations of FIG. 5 with not only onework-piece 3 but rather with two work-pieces 3 a, 3 b. These work-piecesare held by a through clinched connection, with the auxiliary joiningelement 2, 2′ remaining as a “lost plunger” in the work-pieces 3 a, 3 b.The work-pieces 3 a, 3 b are connected together with a relatively highhead pull-out strength and shear strength. The auxiliary joining element2, 2′ is reliably held.

FIG. 7 shows an apparatus corresponding to the representation of FIG. 1.The same parts are provided with the same reference numerals. The onlything which is changed is the plunger 9′ in order the receive theauxiliary joining element 2. It can also be seen from FIG. 7 that onecan press with the auxiliary joining element 2′ onto an auxiliaryshoulder 7′ which is not provided beneath the thread but rather radiallyalongside the thread. As a result of the relatively large materialthickness the thread is however not deformed by the pressure but ratherretains its accuracy with respect to a thread gage.

FIG. 8 shows an embodiment modified relative to FIG. 1 of an apparatusfor the connection of the auxiliary joining element 2 to thesheet-metal-like work-piece 3. The same parts are provided with the samereference numerals while corresponding elements are characterized by thesame reference numerals with a dash.

In comparison to the embodiment of FIG. 1 only the form of the levers22′ and in the die 15 has basically changed. The lever 22′ has, now aspreviously, a short limb 23 with a top side 33 and an end face 25, withthe end face 25 forming the movable undercut regions. The short limb 23is also connected to a long limb 24. This limb 24 however has an outerside 27′ which one can compare with the shoulder 27 in the lever of FIG.1 which in the section of FIG. 8 forms a straight line. In cross-sectionit can naturally also by semi-circularly rounded as can be recognized inFIG. 9. This outer side 27′ does not extend parallel to the pressingdirection 14, but is rather inclined to it and thus includes an acuteangle with the pressing direction 14, with the corresponding wall 34 ofthe die being so directed that the corresponding recess bounded by thewall 34 opens radially upwardly, i.e., towards the plunger 9.Accordingly, the lever 22′ is moved radially outwardly when it islifted. This movement is restricted by a ring 35 which is arrangedbeneath the projection and which is contacted by a lever projection 36at the lower end of the lever 22′ when the lever 22′ has been drawn outupwardly by a sufficient amount. This should be explained with referenceto FIGS. 10 a–10 d. FIG. 10A shows the situation which results when theauxiliary joining element 2 has been pressed into the work-piece 3 andthe corresponding undercut regions 29 have formed. The work-piece 3presses in this arrangement onto the upper side 33 of the lever 22′, sothat the lever 22′ is held fast in the die 15′ and cannot deviateradially.

After termination of the joining process the work-piece 3 is lifted.Since the undercut region 29 basically does not pass through the openingwhich the levers 22′ leave free at their top side, the levers 22′ arelifted with it and are drawn out of the die 15′ in the pressingdirection (in the illustrated embodiment in a vertical direction).During this they wander radially outwardly as can be seen in FIG. 10B.

At the end of this movement the lever projection 36 comes into contactwith the ring 35. These two parts thus together form a security againstdrop-out. The ring 35 is so positioned that at the end of the movementthe opening between two oppositely disposed levers 22′ is precisely solarge that the undercut regions 29 can be drawn out. This can berecognized into FIG. 10 c.

FIG. 10 d shows the state after the work-piece 3 has been fully drawnout. In this instant the levers 22′ can drop back into the die 15′ againso that the recess 19 with the movable side walls formed by the levers22′ is again available for a new joining process.

1. A method of fastening an auxiliary joining element to a sheet metalwork-piece wherein the auxiliary joining element includes a foot,comprising the steps of: providing a die having a recess defined by awall disposed in said die, wherein said wall is interrupted by die partsbeing movable radially inwardly of said wall; retaining the sheet metalwork-piece over said recess; depressing the foot into the sheet metalwork-piece thereby deforming the sheet-metal workpiece into said recessand holding said die parts radially inwardly of said wall therebysimultaneously deforming said sheet metal work-piece into the foot andforming spaced undercuts into the foot for retaining the auxiliaryjoining element to the sheet metal work-piece.
 2. The method as setforth in claim 1, further including the step of forcing portions of thesheet metal work-piece spaced from the undercut into the undercut. 3.The method as set forth in claim 1, wherein said step of depressing thefoot into the sheet metal work-piece is further defined by forming wallsections into the sheet metal work-piece generally parallel to adepressing direction of the foot.
 4. The method as set forth in claim 1,further including the step of forcing said die parts radially outwardlywhile withdrawing the foot from said cavity thereby releasing theundercuts from said die parts.
 5. The method as set forth in claim 1,wherein said step of forming undercuts in the foot is further defined byforming at least three undercuts in the foot.
 6. The method as set forthin claim 1, wherein said step of depressing the foot into the sheetmetal work-piece is further defined by providing the auxiliary joiningelement having an auxiliary shoulder and depressing the auxiliaryshoulder downwardly thereby forcing the foot into the sheet metalwork-piece.
 7. The method as set forth in claim 6, wherein said step ofproviding the auxiliary joining element having an auxiliary shoulder isfurther defined by locating the auxiliary shoulder adjacent the sheetmetal work-piece.
 8. The method as set forth in claim 1, wherein saidstep of retaining the sheet metal work-piece over said recess is furtherdefined by depressing two sheet metal-pieces over said recess.
 9. Themethod as set forth in claim 8, wherein said step of depressing the footinto the sheet metal work-piece is further defined by depressing thefoot into two sheet metal work-pieces.
 10. An apparatus for fastening anauxiliary joining element to a sheet metal work-piece, comprising: aplunger movable in pressing direction for deforming the sheet metalwork-piece; a die defining a recess with a peripheral wall beinggenerally parallel to said pressing direction of said plunger and havingmovable wall sections spaced around said peripheral wall and beingmovable inwardly thereby forming an undercut in the auxiliary joiningelement and the sheet metal work-piece, wherein said plunger includes aholder adapted to engage the auxiliary joining element for driving theauxiliary holding element into the sheet meal work-piece.
 11. Theapparatus as set forth in claim 10, wherein said recess defines an axisand said peripheral wall disposed between said movable wall sections isgenerally parallel to said axis.